Effects of the serotonin-selective reuptake inhibitor paroxetine on locomotion by the larva of the yellow fever mosquito Aedes aegypti

In the simple environment of a Petri dish, larval mosquitoes show a limited behavioral repertoire that can be divided into distinct behavioral states. These states consist of two distinct forms of locomotion, and brief periods of non-locomotion that appear to be correlated with ventilation. Locomotion consists of two distinct gaits, wriggling in which the body flexes from side to side, and non-wriggling locomotion in which the larval body remains linear and apparently is pulled forward by the actions of the mouthparts on the bottom of the Petri dish, possibly associated with feeding. Typically, larvae spend more than 90% of their time performing locomotion, with bursts of wriggling locomotion interspersed with episodes of non-wriggling locomotion. Using motion tracking software (Swistrak) to analyze videos, the serotonin-selective reuptake inhibitor Paroxetine (Paxil^TM, etc.) was found to induce a shift from wriggling to non-wriggling locomotion. Control larvae spent 50 ± 0.3 % of their time wriggling and 50 ± 0.3 % not wriggling (P > 0.88, t-test), while larvae exposed for 24 hours to 1x10^-5mol^.L^-1 paroxetine spent 30 ± 0.4 % of time wriggling and 70 ± 0.4 % not wriggling (P < 0.001, t-test). This change is attributed to a shift from wriggling to non-wriggling locomotion suggesting actions of serotonin on central decision making processes. The number of bouts of wriggling per minute was unchanged (P > 0.55) suggesting that paroxetine reduced the duration of the bursts rather than the frequency of bursts. Exposure to paroxetine also reduced the frequency of wriggles within bursts, from 5.5 ± 0.4 to 3.3 ± 0.3 Hz, but did not influence the velocity during wriggling suggesting actions on central pattern generators of the nerve cord in conjunction with changes in the biomechanics of swimming.  These actions of paroxetine suggest involvement of serotonin in the central control of locomotion at multiple hierarchical levels in this insect.